A computer system typically includes a peripheral component interconnect (PCI) system to interconnect one or more microprocessors with attached external devices. A PCI system includes PCI slots which are disposed in the computer system to be externally accessible so that PCI cards can be inserted therein. Compatible external devices can be plugged into the PCI cards. PCI cards are one type of expansion card. Each PCI slot typically includes a PCI connector and a PCI controller to control the PCI connector and to generate a PCI clock.
The PCI standard specification defines separate expansion PCI connectors for 5-volt signaling and 3.3-volt signaling environments. A location of a physical key in the PCI connector controls which type of signaling environment is to be employed in the PCI system. 5-volt only PCI cards only physically fit into a PCI connector keyed for 5-volt signaling. Similarly, 3.3-volt only PCI cards only physically fit into a PCI connector keyed for 3.3-volt signaling. A third type of PCI card is a universal card which can physically fit into a connector keyed for 5-volt or 3.3-volt signaling. The universal card is designed to adapt to the signaling environment of the PCI connector.
According to the PCI standard specification, the 5-volt signaling environment allows a maximum clock rate of 33 MHz, and the 3.3-volt signaling environment allows a maximum clock rate of 66 MHz. Therefore, conventional computer systems which support 5-volt only (33 MHz) and universal (33 or 66 MHz) PCI cards, must include 5-volt keyed PCI connectors for 33 MHz operation and 3.3-volt keyed PCI connectors for 66 MHz operation. The input/output (I/O) throughput of a computer system is limited by the number of PCI slots in the computer system that can be configured into the computer system's form factor design and the maximum frequency and maximum data width operation of the PCI slots. Thus, there is a high demand for a maximum number of high-performance PCI slots which operate at 66 MHz and can handle 64 bit transfer operations. The more high performance 66 MHz/64 bit PCI slots accommodated into the computer system maximizes the I/O bandwidth potential of the computer system. Nevertheless, many computer systems must also accommodate 5-volt only PCI cards which only operate up to 33 MHz. Conventional computer systems that include 3.3-volt type slots and 5-volt type slots degrade both performance and connectivity.
For reasons stated above and for other reasons presented in greater detail in the Description of the Preferred Embodiments section of the present specification, a computer system is desired which accommodates 5-volt only type PCI cards operating at 33 MHz and universal type PCI cards operating at 66 MHz in the same type PCI slots.